Titanium dioxide nanotube (TONT) arrays grown by electrochemical anodization were used as supports for alloyed PtNi catalysts which were developed by a cosputtering technique. The as-deposited PtNi on TONT had poor oxygen reduction reaction (ORR) activity with a significant overpotential, while the ORR activity of annealed PtNi/TONT was improved. The maximum ORR activity, with a significant increase (120 mV) of the half-wave potential, was achieved for 400 degrees C annealed PtNi/TONT. This favorable enhancement was attributable to two factors. The first is the formation of a PtNi alloy catalyst, which resulted in the modification of the d-electronic structure of Pt, leading to the favorable adsorption of O-2. The second factor is a strong metal-support interaction (SMSI) effect. When the high-temperature thermal treatment was conducted in the reducing gas (hydrogen gas) on the PtNi/TONT, there was electron transfer from the reduced support to the Pt catalyst. This caused a change of the work function that altered the electronic properties of surface Pt atoms (X-ray photoemission spectroscopy measurement), leading to an enhancement of the ORR activity. To investigate the SMSI effect further, the thin Pt/TONT samples were prepared and used for the investigation of the interfacial region between the TONT support and the Pt catalyst. (C) 2008 The Electrochemical Society.